Announce: Moving session startup from upstart to systemd

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Announce: Moving session startup from upstart to systemd
Martin Pitt martin.pitt at ubuntu.com
Sun Jul 31 15:09:10 UTC 2016

Hello Ubuntu developers!

What is this about?
===================
As discussed at UDS [1] we are moving away from using upstart to start graphical desktop sessions, towards systemd (and D-Bus activations in some cases where it's appropriate). Two weeks ago Sebastien Bacher, Iain Lane, Ted Gould, and me had three-day sprint where we converted most services of the Ubuntu session, and before/after I was working on the necessary infrastructure in systemd and upstart, and converted/checked most other flavors. This is now ready to land and get wider testing.

Landing
=======
The main "switch" is being done in the Exec= lines of the corresponding /usr/share/xsessions/*.desktop. Display managers use these to show which sessions are available and to know how to start them. This happend in ubuntu-desktop 3.18.1.2-1ubuntu5 [2] and xubuntu-default-settings 16.10.1 [3], which just landed in Yakkety.
Other desktops like Kubuntu, Lubuntu etc. don't use upstart and thus have not been converted; if you are interested in this, please talk to me, or Sebastien/Iain (as I will be on holidays from next Wednesday on).

With this, about half of your session will then be driven by systemd units, check "systemctl --user status" and "initctl list|grep run" before and after. unity itself, hud, and indicators are still run by upstart -- their conversion is being prepared in CI train [4]. If you want, you can get systemd units for those by enabling the staging PPA [5] and installing the "systemd-graphical-session" package -- this package and the PPA will become empty over time as things are being landed.

Note that some upstart jobs like "dbus" or "gnome-session" will still run -- their overrides change the program to be run to "sleep infinity" so that they become stub jobs. This is necessary as we still want their dependencies to get started until the transition gets finished, i. e. all reverse upstart job dependencies of those stubs
get converted.

Next steps
==========
We now need to provide systemd units for the remaining jobs. On desktop (all flavors) it's pretty much complete with the PPA, the main missing piece is to convert the unity greeter session. This is also the only actual reverse dependency of "upstart" -- in a VM with the PPA you can force-purge upstart and run a desktop entirely without it. Indicators will be missing in the Unity greeter session in lightdm, but they are not essential.

The bulk of the remaining work is to convert the Touch packages and ubuntu-app-launch. There are also some workarounds in place for missing functionality in systemd. All remaining work is being tracked
in the blueprint [1] and in bugs tagged with "systemd-session" [6].

D-Bus user session
==================
The upstart → systemd transition also depends on/contains a separate transition from a session-centric to user-centric D-Bus daemon and services. A lot of D-Bus services like pulseaudio, gvfs, SSH agents, keyring daemon etc. are useful to other parallel (non-graphical, i. e. VT or ssh) logins of the same user, so these will now continue to live
until the last session of a particular user gets stopped. See e. g. [7] for some details. This is being achieved by installing the "dbus-user-session" package, on which the converted ubuntu-session/xubuntu-default-settings packages now depend.  This also caused some fallout mostly in policykit, gnome-keyring, and AppArmor profiles, which all got fixed in Yakkety. There are no other known issues with those at the moment, so if you notice anything else please file a bug about it and tag it with "systemd-session".

Contingency plan
================
If this causes trouble, this can be switched back by reverting the Exec= lines, in these two commits:

  http://bazaar.launchpad.net/~ubuntu-desktop/gnome-session/ubuntu/revision/343#debian/patches/50_ubuntu_sessions.patch
  (this commit has other changes, just the hunk in this file)

  http://bazaar.launchpad.net/~xubuntu-dev/xubuntu-default-settings/trunk/revision/621

As an alternative, unless you removed upstart, you temporarily switch back to the upstart jobs by editing /etc/X11/Xsession.d/00upstart and replacing the line

    if [ "${1#*.target}" != "$1" ]; then

with

    if false && [ "${1#*.target}" != "$1" ]; then

(i. e. effectively disable the "if").

References
==========
[1] https://blueprints.launchpad.net/ubuntu/+spec/convergence-y-replace-upstart
[2] https://launchpad.net/ubuntu/+source/gnome-session/3.18.1.2-1ubuntu5
[3] https://launchpad.net/ubuntu/+source/xubuntu-default-settings/16.10.1
[4] https://requests.ci-train.ubuntu.com/#/ticket/1710
[5] https://launchpad.net/~ubuntu-desktop/+archive/ubuntu/systemd-session
[6] https://bugs.launchpad.net/ubuntu/+bugs?field.tag=systemd-session
[7] https://lists.debian.org/debian-devel/2016/07/msg00484.html

Thanks,

Martin
--
Martin Pitt                        | http://www.piware.de
Ubuntu Developer (www.ubuntu.com)  | Debian Developer  (www.debian.org)

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перевод на русском тут https://forum.ubuntu.ru/index.php?topic=280463.0